The present invention relates to lathes for cutting aspherical surfaces or, more precisely, truncated aspherical surfaces on workpieces, for example, for the production of lenses, contact lens moulds, reflectors and the like.
Typically, a lathe for cutting spherical surfaces for such purposes as contact lenses, comprises a headstock mounted on a slideway on the lathe bed enabling the headstock to move parallel to its spindle axis under the control of a lead screw journalled in a fixed bearing and rotatable, for example, by a handle. A tool holder or tool post is mounted on a radius cartridge so that a cutting tool clamped in the post can swing at a preselected radius about an axis disposed perpendicularly to the rotational axis of the headstock spindle and in the same plane as the spindle axis. By advancing the headstock in steps towards the cutting tool, which is swung about the axis of the radius cartridge, a workpiece clamped in a chuck or collet at the end of the spindle can be cut to either a truncated concave or convex spherical surface depending on the position of the tool post with respect to the axis of the radius cartridge. The purpose of advancing the headstock, in this instance, is for removing increments of material from the workpiece and it is important to note that, whilst the radius cartridge is rotated, the headstock is maintained stationary.
Requirements have arisen for cutting aspherical surfaces, such as, truncated elipsoids or other conic sections. For example, such surfaces are desirable in the case of contact lenses in order accurately to conform the central part of a contact lens surface to an eyeball, which is not normally spherical, and to provide edge lift at the periphery of the contact lens to permit access of tear fluid to the underneath of the lens for lubrication purposes. In such cases, the objective is to "flatten" or "steepen" the surface being cut, away from the shape of a true sphere, as the diameter of the surface increases.
Two main arrangements are currently used to produce aspherical surfaces on workpieces. The first consists in linking the tool post of a lathe of the type described above to a cam or other mechanism so that the radius at which the tool is cutting changes as the radius cartridge rotates. Such an arrangement is described in U.S. Pat. No. 4,114,486. The disadvantage of this arrangement is that different cams or complicated adjustments are required for aspherical surfaces of different eccentricities so that it tends to be inflexible. Moreover, because of the small movements involved, movment of the tool post to change the cutting radius is difficult to control and tends to be unstable resulting in a relatively poor finish to the cut surface.
The second main arrangement currently used employs a lathe with a normal cross-slide, instead of a radius cartridge, and the tool position is controlled by means of a pair of slides movable relatively at right angles to one another. Thus, by controlling the relationship of one axis with respect to the other an aspherical surface can be cut. It is usual to control the positions of the two slides either by means of an electrical servo or stepping motor. Either means generally requires microprocessor control and the input of a programme which specifies the shape of the aspherical surface to be cut. Again, this arrangement tends towards inflexibility since, to change the shape of the aspherical surface being cut, it is necessary to change the programme for the microprocessor control. Also, such form of control is expensive.
Other arrangements for producing aspherical surfaces on workpieces are described in FR-A-No. 2378607 and 2212772, EP-A-No. 44207, U.S. Pat. Nos. 4,274,313, 4,068,413 and 3,664,065 and GB-A-No. 1290685.